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Comment by jakedata | Hacker News Reader

Just sitting here speculating, not even a web search yet...

In the USA we have both ground and neutral wires at every outlet along with the hot (120VAC nominal) wire.

Why couldn't we raise the ground wire to +12VDC (or whatever) with regard to the neutral? The ground would have a very sensitive detector so that if electrical conditions exceeded some preset threshold it would collapse to zero nearly instantly, allowing the circuit protection to function as expected.

You could make a plug that only has a neutral and ground pin to tap into the low voltage side, or use all three pins to provide low power standby mode.

Now I'll do a search to find out that it is impossible because of a, b or c reasons or that it has already been patented somewhere and abandoned. Yay Internet...

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The GND (or Earth/GND) is there for safety and equipotential bonding. It’s not meant to carry current and running current through it will trigger your RCD, because that’s how a ground fault is detected. And detecting that is important so when you trip with your hairdryer into the bathtub you won’t kill yourself.

Even if plug is polarized (plug goes in only in 1 orientation), you can't assume any of the high voltage wires to be touch-safe. Wiring mistakes (swaps) can & do happen.

In-house AC systems are designed to remain safe when that occurs. But it prevents using any of those wires in ways like what you suggested.

One problem is that many older houses don't have grounding. Half my outlets are ungrounded and I use GFCI to get three-prong plugs. Or the grounding is done through pipes and conduit which aren't suitable for running current.

The standards for grounding make them unsuitable for running current. The ground wire in box are frequently bare. Metal boxes are grounded. The center screw is grounded. If you run current over the ground, those are live.

You have invented a way to electrocute lots of people.

Ground and neutral are tired together at the electrical panel.

That's why this could work - the panel is the logical place to insert the DC supply. If ground and neutral were bonded in multiple places then it would just be a DC short.

They are bonded there for safety reasons and that bond needs to be low impedance.

The reason is that you don’t want to allow a fault which puts a line voltage (120VAC) onto the chassis of a piece of equipment waiting for a human to come along and complete the circuit to ground. So, you use the protective earth to connect to the chassis, such that if a power line ever did contact it, it would be a dead short to ground, which would trip the OCPD (over-current protection device, typically a circuit breaker), clearing the fault.

If you replace this safety mechanism with something that’s high-enough impedance to maintain a 12VDC differential, you’ve eliminated this important safety mechanism in your AC distribution system.

I think this would also make another possible fault more dangerous: a break in neutral somewhere between an AC device and the panel.

If there was also a DC device on the same side of the break as the AC device, then you'd have a possible return path for the AC current that goes through the DC device, the ground wire, and the DC power supply.

In addition to that, would it also cause problems with GFCI?

The 12 VDC return current would be on the neutral. I'd guess that a steady DC current on the neutral wouldn't cause problems, but whenever a DC device is switched on or off, or you use a DC device that has a fluctuating instantaneous power need, that should induce current in the GFCIs sensing coil that won't be canceled out by an opposite current induced from the load wire.

I don't know if that mismatch would be long enough or large enough to trip the GFCI.

GFCIs will trip at ~5mA of current imbalance, so your point is an additional excellent one.

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